In order to acquire and observe the waveform data of the optical signal modulated by a high-speed repetitive signal, for example, a waveform observation device 10 shown in FIG. 10 is used.
In this waveform observation device 10, an optical sampling pulse Ps having a narrow pulse width and a repetitive period Ts (=N·Tx+ΔT) longer by a predetermined value (offset delay time) ΔT than a value N times (N is an arbitrary integer not less than 1 such as 100 or 1000) of the repetitive period Tx of the waveform of an input measured optical signal P is generated by optical sampling pulse generating means 11.
The optical sampling pulse Ps generated by the optical sampling pulse generating means 11 is input to an optical sampling unit 12 together with the measured optical signal P.
In this optical sampling unit 12, the pulse light obtained by sampling the measured optical signal P by the optical sampling pulse Ps is subjected to photoelectric conversion into an electrical pulse signal Eo and output to an analog/digital (A/D) converter 13.
This A/D converter 13 converts the amplitude strength of the electrical pulse signal Eo into digital data and causes to store it in a waveform data memory 14.
A series of waveform data stored in this waveform data memory 14, after being read by display control means 15, is displayed as a waveform of the measured optical signal P on a display unit 16.
In the waveform observation device 10 of thus sampling scheme, as shown in (a) of FIG. 11, the sampling timing by the optical sampling pulse Ps is shifted ΔT time as shown in (b) of FIG. 11 each time the repetitive waveform of the measured optical signal P is input N times continuously. Therefore, a series of the waveform data, obtained by sampling the waveform of the measured optical signal P with a high resolution at a remarkably low sampling rate as compared with the period Tx, can be observed on the screen of the display unit 16.
Thus sampling scheme waveform observation device 10 is disclosed, for example, in Patent Document 1 described below.
The observation modes required of this waveform observation device 10 include a persistence mode and an averaging mode.
The persistence mode is the one in which the operation is repeated to sample the measured optical signal P and display the acquired data on the screen of a display unit for a predetermined time so that the measurement waveform is displayed based on the incidental image thereof, and the change in the waveform of the measured optical signal can be observed substantially in real time.
The averaging mode, on the other hand, is the one in which the measured optical signal P is sampled and the waveform data acquired for a plurality of data acquisition periods are averaged out and the averaged waveform is displayed. This mode makes possible the waveform observation with noise components removed.
Unless the operation of sampling the measured optical signal P is started from the same phase position of the repetitive waveform thereof, the waveform displayed is inconveniently displaced each time along the time axis in the observation mode in which the waveform of the measured optical signal is displayed by the incidental image thereof.
Also, in the averaging mode, the averaging process cannot be correctly executed and the waveform cannot be correctly reproduced, while at the same time making it impossible to correctly grasp the waveform phase and the size of the amplitude variation.
For this reason, the repetitive period of the waveform of the measured signal or the frequency (bit rate) of the signal itself is required to be known in advance.
In some cases where the correct value, not the approximate value, of the repetitive period of the waveform or the frequency of the measured signal to be observed is unknown, however, the correct sampling period cannot be set for the waveform of the measured signal to be observed, thereby posing the problem that the waveform cannot be observed as desired.
Also, in this type of the waveform observation device, an optical mixer or the like for generating the optical sampling pulse having a narrow width or mixing the light with each other is required, thereby posing another problem that the whole device including the display unit is complicated and increased in cost.
In view of this, the present inventor, in order to solve this problem, has proposed a repetition frequency detection method of a measured signal as disclosed in Patent Document 2 described later as a prior application in Japan.
Next, the principle of the measured-signal repetition frequency detection method disclosed in Patent Document 2 will be explained.
For the present purpose, the measured signal is assumed to be a sinusoidal wave of a single frequency Fx, and the frequency component of the signal Sx obtained by sampling this signal with a provisional sampling frequency Fs is studied.
In the case where the sampling pulse is an ideal one having an infinitely small width, the frequency component thereof has each spectrum of frequency n·Fs as shown in FIG. 12 (n=0, 1, 2, . . . ).
The signal Sx obtained by sampling using this sampling pulse, therefore, contains components including the difference and the sum between the frequency Fx of the measured signal and each frequency n·Fs.
Among these components, the one having the lowest frequency, as shown in (a) and (b) of FIG. 13, is the difference frequency with the spectrum component of the frequency n·Fs nearest to the frequency Fx or the difference frequency with the spectrum component of the frequency (n+1)·Fs. This difference frequency Fh can be expressed as follows:Fh=mod[Fx, Fs] . . . (in the case where mod[Fx, Fs]≦Fs/2)Fh=(Fs/2)−mod[Fx, Fs] . . . (in the case where mod[Fx, Fs]>Fs/2)where the symbol mod[A, B] indicates the remainder after dividing A by B.
This difference frequency Fh is Fs/2 at maximum, and therefore, can be easily extracted by use of a low-pass filter having the upper limit band of Fs/2.
Now, the change δFh in the difference frequency Fh due to a minuscule change δFs of the sampling frequency Fs is given by the following equation obtained by differentiating the difference frequency Fh with the frequency Fs.δFh/δFs=−quotient[Fx, Fs] . . . (in the case where 0<mod[Fx, Fs]<Fs/2)δFh/δFs=1+quotient[Fx, Fs] . . . (in the case where mod[Fx, Fs]>Fs/2)where the symbol quotient[A, B] indicates an integral quotient obtained by dividing A by B.
From this result and the relationmod[Fx, Fs]=Fx−Fs·quotient[Fx, Fs]between the quotient and the remainder, the frequency Fx of the measured signal can be determined from the following arithmetic operation.Fx=Fh−Fs·δFh/δFs . . . (in the case where 0>δFh)Fx=−Fh+Fs·δFh/δFs . . . (in the case where 0<δFh)
FIG. 14 is a flowchart showing an example of the steps of the repetition frequency detection method of the measured signal described above.
First, the measured signal is sampled with a provisional sampling frequency Fs (step S1), and among the signals obtained by this sampling, the frequency Fh of a specified signal appearing in the band not more than one half of the sampling frequency Fs is detected (step S2).
Then, the sampling frequency is changed by a minuscule amount ΔFs (for example, 1 Hz) (step S3), and the corresponding frequency change amount ΔFh of the specified signal is detected (step S4).
Then, the sampling frequency Fs with the frequency change amount ΔFs thereof and the frequency Fh of the specified signal with the frequency change amount ΔFh thereof are substituted into Equation (1) below thereby to calculate the repetition frequency Fx of the measured signal (step S5).Fx=Fh−Fs·ΔFh/ΔFs . . . (in the case where 0>ΔFh)Fx=−Fh+Fs·ΔFh/ΔFs . . . (in the case where 0<ΔFh)  (1)
As a result, in the case of a system for acquiring and observing the waveform information, the waveform information of the measured signal can be acquired and observed accurately by executing the aforementioned frequency detection process for the measured signal and setting a sampling frequency Fs corresponding to the frequency Fx obtained thereby.
Now, the error contained in the detected repetition frequency described above is studied. The definition above shows that the value δFh/δFs is an integer, and therefore, in the case where the value ΔFh/ΔFs obtained by actual measurement is not an integer, the measurement error can be eliminated by rounding off the fractions of the value to the nearest integer.
Also, since the sampling frequency is a value given by the system itself, no error is generated.
Further, the measurement error of the frequency Fh of a specified signal is determined by the resolution of the digital signal processing such as the fast Fourier transform (FFT) and can be reduced easily to several Hz or less.
These facts indicate that the calculation error of the repetition frequency Fx of the measured signal can also be reduced to the accuracy of not more than several Hz.
This error is, for example, 10−10 for the repetition frequency of 10 GHz. Thus, the repetition frequency of the measured signal can be detected with a very high accuracy.
Furthermore, the foregoing description is based on the assumption that the measured signal is a sinusoidal wave of a single frequency Fx, and the measured signal to be actually observed normally contains a plurality of frequency components.
Specifically, in the case where the measured signal is a signal modulated by the data according to the non-return to zero (NRZ) scheme, a multiplicity of frequency components Fx(i) may exist down to the lower frequency limit corresponding to the period (waveform repetition period) equal to the code length of the particular modulated data, and the level of each frequency component depends on the pattern of the modulated data.
In the case where the modulated data is a 2-bit data (10) of 10 Gbps, for example, the frequency component of 5 GHz one half of the bit rate and a harmonic component thereof exist so that the frequency component one half of the bit rate is highest in level.
In the case where the modulated data is a 10-bit data (1111100000) of 10 Gbps, on the other hand, the frequency component of 1 GHz one tenth of the bit rate and a harmonic component thereof exist, and the frequency component of 1 GHz one tenth of the bit rate is highest in level.
Also, in the case of a pattern which is not so simple that the periods of 1 and 0 of the same length alternately appear with the duty factor of 50% as described above but in which the periods of 1 and 0 appear a plurality of times within one code such as (1100011100), the level of the frequency component of 2 GHz one fifth of the bit rate is higher than that of the frequency component one half of the bit rate or the frequency component corresponding to the period equal to the code length.
As described above, the frequency of a specified signal with a frequency component higher in level, though not coincident with the repetition frequency of the signal waveform, is detected more advantageously from the viewpoint of S/N and the repetition frequency is determined more advantageously with a higher accuracy from the frequency of this specified signal.
Also, Patent Document 2 discloses a waveform observation system including a sampling apparatus using the measured-signal repetition frequency detection method described above.
FIG. 15 shows the configuration of a waveform observation system 20 including the sampling apparatus using the measured-signal repetition frequency detection method described above.
This waveform observation system 20 is configured of a sampling apparatus 21 and a digital oscilloscope 60.
In the sampling apparatus 21, the measured optical signal P input from an input terminal 21a is sampled by an optical sampling unit 26 with a sampling pulse making up an optical pulse having a narrow width generated from a sampling pulse generating unit 24 based on the clock signal C by the signal generating unit 24 thereby to acquire a pulse signal Eo as the waveform information thereof.
The digital oscilloscope 60 stores and displays the waveform information obtained by the sampling apparatus 21.
This sampling apparatus 21 has the manual setting mode designated in the case where the repetition period of the waveform to be observed is accurately known and the auto setting mode designated in the case where the repetition period of the waveform to be observed is unknown or only approximately known. The manual setting mode or the auto setting mode can be selectively designated by the operation of an operating unit (not shown).
Furthermore, the clock signal C and the trigger signal G generated by the signal generating unit 24 can be output outside through a clock output terminal 21b and a trigger output terminal 21d, respectively.
In similar manner, the pulse signal Eo from the optical sampling unit 26 is adapted to be output outside through a sample signal output terminal 21c. 
These output terminals 21b to 21d of the sampling apparatus 21 are connected to an external clock input terminal 60a, a first channel input terminal 60b and a second channel input terminal 60c, respectively, of the digital oscilloscope 60.
The digital oscilloscope 60 has the external clock synchronization function to execute the A/D conversion process of the signal input from the channel input terminals 60b, 60c in synchronism with the clock signal input to the external clock input terminal 60a, the external trigger function to store, as waveform data for each channel, the data obtained by the A/D conversion process during the lapse of a predetermined time (depending on the display width, the number of display points, etc. along the time axis described later) from the timing when the voltage of the input signal to an arbitrary designated input terminal or the trigger input terminal has exceeded an arbitrarily set threshold value in a predetermined direction, and the waveform display function to display the stored waveform data on the time axis. As this waveform display mode, any one of the persistence display mode and the averaging display mode described above can be selected as desired.
Next, the operation of the waveform observation system 20 described above will be explained.
First, as shown in (a) of FIG. 16, for example, a measured optical signal P of a substantially rectangular wave having the duty factor of 50% is input to the input terminal 21a, and the information corresponding to the approximate repetition period Tx′ (frequency Fx′) and the sampling offset delay time ΔT is designated by a parameter designation unit 22 while at the same time designating the auto setting mode through the operating unit (not shown).
The arithmetic unit 23, based on the designated approximate repetition frequency Fx′ and the offset delay time ΔT, calculates the provisional sampling frequency Fs′ and the trigger frequency Fg′, which are then set in the signal generating unit 24.
Furthermore, in the case where the auto setting mode is designated without designating the repetition frequency Tx′, the arithmetic unit 23 performs the arithmetic operation with a specified value such as 10 GMHz as the repetition frequency Fx′.
As a result, the clock signal C having the provisional sampling frequency Fs′ is output from the signal generating unit 24.
In the optical sampling unit 26, the measured optical signal P is sampled at the sampling frequency Fs′, and the pulse signal Eo obtained by this sampling is input to a specified signal frequency detector 27.
The specified signal frequency detector 27 detects, among the frequency components contained in the pulse signal Eo obtained by the sampling operation thereof, the frequency Fh′ of a specified signal made up of the highest frequency component appearing in the band of not more than one half of the sampling frequency.
In the case of the waveform of this optical signal, the spectrum of the optical sampling pulse Ps used for sampling appears at intervals of the frequency Fs′ as shown in FIG. 17, while the spectrum of the waveform of the optical signal S appears at intervals of the frequency Fx. In addition, the spectrum of the higher-order appears in the lower-level.
In the specified signal frequency detector 27, therefore, the difference frequency Fh′ between the lowest-order frequency Fx and the sampling frequency component n·Fs′ nearest to the frequency Fx is determined as the frequency of the specified signal and output to a repetition frequency calculation unit 28.
The frequency Fh′ of the specified signal for the provisional sampling frequency Fs′, once acquired as described above, is stored in the repetition frequency calculation unit 28 which in turn instructs the signal generating unit 24 to change the sampling frequency by a minuscule amount (for example, 1 Hz).
In response to this instruction, the signal generating unit 24 changes the provisional sampling frequency for the measured optical signal P by a minuscule amount ΔFs. With this change, the frequency of the specified signal detected by the specified signal frequency detector 27 is changed by ΔFh. From this change amount, the repetition frequency Fx of the waveform of the measured optical signal P is calculated according to the equation described below and set in the arithmetic unit 23.Fx=Fh′−Fs′·ΔFh/ΔFs′
The arithmetic unit 23, based on the accurate repetition frequency Fx calculated by the repetition frequency calculation unit 28, calculates the regular sampling frequency Fs and the trigger frequency Fg in exact correspondence with the input signal, and sets the result in the signal generating unit 24.
As a result, the clock signal C and the optical sampling pulse Ps having a period equal to N·Tx+ΔT for the repetition period Tx of the waveform of the measured optical signal P are generated as shown in (b) and (c) of FIG. 16.
Then, the measured optical signal P is sampled by the optical sampling pulse Ps, and the pulse signal Eo obtained by this sampling is input to the first channel input terminal 60b of the digital oscilloscope 60 through the sample signal output terminal 21c from the optical sampling unit 26 as shown in (d) of FIG. 16.
Also, a trigger signal G having a period equal to the period of the waveform of the envelope connecting the peaks of the pulse signal Eo as shown in (b) of FIG. 18 is generated from the signal generating unit 24, and through the trigger output terminal 21d, input to the second channel input terminal 60c of the digital oscilloscope 60.
Furthermore, (a) of FIG. 18 shows the time axis, in compressed form, of the waveform shown in (d) of FIG. 16.
The digital oscilloscope 60 executes the A/D conversion process for the pulse signal Eo in synchronism with the clock signal C, and the data on the envelope connecting the peak points of the pulse signal Eo are sequentially output as optical signal waveform data, and from the timing when the trigger signal G exceeds the trigger level in a predetermined direction, begins to acquire the waveform data thereof.
As shown in FIG. 19, therefore, the waveform of the measured optical signal P is displayed as an incidental image at points with the intervals of the offset delay time ΔT on the screen of the digital oscilloscope 60.
In the digital oscilloscope 60 which begins to acquire the waveform data at each timing when the trigger signal G exceeds the trigger level in a predetermined direction and displays an updated waveform, the sampling frequency and the trigger frequency of the sampling apparatus 20 accurately correspond to the repetition frequency of the waveform of the input optical signal P as described above, and therefore, an always-stable waveform observation is made possible without any displacement of the waveform display position.
The foregoing description represents a case in which the waveform to be observed is a rectangular wave having the duty factor of 50% and the lowest-order specified signal is at maximum level.
In the case where the waveform of 10 bits of the NRZ data (1100011100) is repeated at the bit rate of 10 Gbps, for example, the repetition frequency Fx thereof is 10/10=1 GHz. Taking the level of each frequency component contained in the waveform into consideration, however, the component of 10/2=2 GHz which is twice as high as the 1-GHz component is larger.
This is also the case with the signal contained in the pulse signal Eo, and as described above, the frequency component of the lowest-order specified signal for the repetition frequency Fx equivalent to one waveform period is so low in level that the frequency may not be calculated accurately.
Even in such a case, the specified signal frequency detector 27 selects the signal component of the highest level as a specified signal among the signal components within the band of not more than one half of the sampling frequency, and detects the frequency thereof. Thus, no accuracy reduction occurs.
Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 2002-071725
Patent Document 2: Jpn. Pat. Appln. KOKAI Publication No. 2006-3327